In television broadcasting (including wireless broadcasting and cable broadcasting) a television video signal may be scrambled at the program broadcasting side and processed so as to be sent out in such a way that the picture may not be normally be received as sent. The video signal must be restored to a normal state by a key signal at the picture receiving side, so that the program may be viewed and heard. The present invention relates to a method for developing a key signal for descrambling television broadcasting in which the key signal for restoring the picture is added a plurality of times to one field so as to protect the key signal from external disturbances such as data error, noise and the like, thereby making it possible to completely effect descrambling.
In conventional TV broadcasting services, either by wireless or cable, programs can be watched on normal, standard TV sets, within respective area covered by service networks, by reproducing TV pictures as received. However, it is necessary to make it possible to view TV programs only on TV sets owned by specific subscribers, but not by others with respect to paid TV broadcasting services. Particularly, in CATV broadcasting services capable of providing various programs, the operation of such CATV stations is dependent on the fees paid for programs, so that the segregation of subscribers is essential. For that reason, transmitters of programs require a scrambling method which manipulates the pictures transmitted so that they cannot be reproduced as normal pictures if received as they are.
Firstly, a CATV system will be outlined hereunder with reference to FIG. 1.
This system has a unique chargeable program broadcasting function.
FIG. 1 shows the entire CATV system. This system comprises a center 1 and thousands and thousands of terminal equipment units 28 connected therewith by means of coaxial cables. From the center 1 is laid a trunk line cable 3 on which are installed at predetermined points a trunk line amplifier 4 and a branch box, respectively. From this branch box 5 are drawn a plurality of branch cables 6, each having a branch amplifier 7 and a tapoff 8, respectively. From each tapoff 8 are drawn plurality of sub-branch cables 9; each terminal thereof is drawn into a household 2. The terminal equipment 28 in each household 2 comprises a main box 10, a TV set 11 and a control box 12. The terminal of a sub-branch cable 9 is connected to the main box 10, the TV set 11 and the control box 12. That is, the center 1 and every terminal unit 28 in subdivisioned households are inter-connected in a Christmas tree manner.
Outside the aforementioned center 1 is erected a receiving antenna 13, and this is connected to a demodulator in a source group 14. In this source group are a video-disc player 16, video-tape recorder 17, studio 18 and so forth. A modulation transmitting part 19 that receives signals from the source group 14 comprises two systems: one consists of an IF modulator circuit 20, a scrambling circuit 21 and an up converter circuit 11; and the other, an IF modulator circuit 23 and an up converter circuit 24. Respective outputs from the up converter circuits 22 and 24 are connected to the trunk line cable 3. The trunk line cable 3 is also connected to a data transmitter/receiver set 25 that communicates with each main box 10. To the data transmitter/receiver set 25 is connected a computer 26 to which is connected perpheral equipment 27 including a printer, display, etc.
The operation of this CATV system will now be described; turning ON the TV set 11 and operating the control box 12 to select a desired channel enables pictures to be supplied to the TV set 11 through conversion, by means of the main box 10, of a frequency for said channel into that for an idle channel. Types of channels selectable selectable by means of the control box 12 comprise:
(A) retransmissions for receiving programs as they are; PA1 (B) self-sustained programs (free of charge); and PA1 (C) chargeable programs.
Each group has several channels, respectively, thus making a total of some 20-30 or more selections available.
(A) Retransmission
Electromagnetic waves received by the receiving antenna 13 are demodulated by means of the demodulator 15 and delivered to the modulation transmitting Section 19. The signals is then modulated in the IF modulator circuit 23 and the modulated signal is raised to a specific frequency in the up converter circuit 24. This signal, having been modulated and positioned to a specific channel, is transmitted by way of the trunk line cable 3, branch cables 6 and the sub-branch cables 9 to the households 2 and is received by the TV sets 11 through the medium of the terminal equipment 28.
(B) Self-sustained Programs:
These programs include weather forecasts, newswires, stock quotations and the like. Recorded programs regenerated by the video-disc player 16 or the video-tape recorder 17 and live programs produced in the studio 18 are transmitted to the trunk line cable 3 upon being modulated and subsequently converted into a frequency for a specific channel in the IF modulator circuit 13 and the up converter 24, respectively. These programs are free of charge regardless of the number of times of reception or hours thereof, and each household may receive these programs upon monthly payment of a base fee.
(C) Chargeable Programs
These programs include newly produced motion pictures and other specific programs. Recorded programs are regenerated on the video-disc player 16 or the video-tape recorder 17, and live programs produced in the studio 18 are modulated in the IF modulator circuit 20, followed by adding to the videosignal a specific synchronizing signal in the scrambling circuit 21, to manipulated the picture signals so that they will not appear as normal pictures upon reception of the same as they are. The frequency of the signal is then raised in the up converter circuit 22 to that for a specific channel and the signal is then sent to the trunk line cable 3. Each household 2 desiring to watch this chargeable program may be able to do so by demodulating the received signals into normal video signals in the main box 10. For reception of chargeable programs, the fees are calculated on a pre-established basis and are added to the monthly base fee on a bill.
The aforementioned center 1 and the terminal equipment 28 of every household 2 are connected by means of coaxial cables, but unless the reception of pictures on certain channels by households 2 is periodically verified, the fair operation of a CATV system cannot be expected because, if not, discrimination between chargeable and free of charge accounts based on programs received is not feasible. For this purpose the data transmitter/receiver set 25 is provided determine the signals being received during a retrieval time (so-called polling) by transmitting a retrieval signal at a certain time interval, calling main boxes 10 in every terminal unit 28 using a unique address number assigned to each main box 10. Each main box 10, in response to the polling signal, generates an answer as to the channel through which pictures are being received, and transmits it back to the data transmitter/receiver set 25. The data received by the data transmitter/receiver set 25 are put through data processing by the computer 26 and are displayed on or printed out by the perpheral equipment 27. Polling is carried out at a certain time interval (several-tens of seconds), thus making it possible to sum ratings, etc. at once. Some programs invite participation by the audience; in which the audience may be able to answer, while watching on the TV sets 11, the question asked in the program by operating the control box 12. The answers (data) are sent to the center 1 through the coaxial cables.
For certain programs in conventional TV broadcasting, video signals transmitted from the center 1 are scrambled (privacy transfer). In order to receive these scrambled pictures as normal, a descrambling process is required at each terminal unit 28 to demodulate the same into regular picture signals. If the scrambling process is primitive and very easily descrambled, wiretapping may not be prevented. A high level scrambling method for preventing ready wiretapping has been in demand for the operation of TV signal transfer systems.
There has been often used a grey-sync method in scrambling in accordance with the prior art. However, there was a drawback in that the quality of the picture was worsened when descrambled in accordance with this method and the decoded signal was easily analyzed so as to obtain the information therein. Under these circumstances, the present inventors have proposed a scrambling method for a video signal, as disclosed in U.S. Ser. No. 509,004, in which only the horizontal synchronizing signal is level-compressed, and a decode (key) signal is digitized so as to be added to the horizontal synchronizing signal in the video signal, while this digitized decode signal is analyzed at the receiving side so as to restore the picture. In accordance with this method, however, there may be caused a data error at the time when the key signal for descrambling is received, or an error may caused in the decode signal due to external disturbance, noise or the like, so that it may become impossible to restore the picture to the normal state. Thus, this method was not entirely suitable in practical use.